1. Field of the Invention
The present invention relates to an apparatus for measuring features of liquids which influence their dielectric characteristics, particularly the composition of the liquid, i.e. the ratio of components.
This measurement is used for example for observation and automatic refilling of the alcohol content in alcohol-water-mixtures, which are used in printing equipment as the so called "fountain-solution".
2. Description of the Prior Art
Prior art devices use a gauge or float valve which reacts to the specific weight of the mixture in accordance with the alcohol content, and, according to its floating level, actuates a switch. One of its shortcomings is the sensitivity to dirt, air bubbles, and mechanical movement. Moreover, an instantaneous exact reading of the measured value is difficult to obtain.
There are different methods which utilize the change of the dielectric constant to evaluate the water content of a material. German OS 3507507 describes a capacitive probe for use with fountain solution, this however only measures the filling level and not the composition of the liquid.
OS-DE 3518186 describes a method for evaluating the water content in alcohol using an approach based on capacity; OS-DE 2002168 describes an electrode probe for evaluation of a water-oil mixing ratio. OS-DE 3433740 and 2436344 describe probes for measuring the water content in butter. All the above methods are based on the evaluation of capacitance which appears at the electrodes of a capacity probe. These conventional techniques are not, however, suitable for measuring the mixing ratio when the water content represents the major portion of the mixture, and additionally when the water's electric conductivity is increased by certain added salts or buffering chemicals typically used in fountain-solutions, because of the resultant errors. Furthermore, having a high percentage of water causes the mixture to have a high dielectric constant (water has about 80), and this dielectric value is reduced only in a small amount by the added alcohol, relative to the high initial value. The commonly used values of alcohol percentages in fountain solutions are 3 to 25%.
Another disadvantage of conventional approaches is that contamination such as oil or ink residue will stick to the electrodes' surface and make the measurement inaccurate due to their own dielectric constant. Oil has an extremely small dielectric constant of about 3 and thus functions like an isolator in series with the capacitive path. The same happens with gas-bubbles which appear from fresh water and tend to stick the electrodes.